In recent years, the new concept of "stem cell plasticity", and the hope that stem cells could be used to repair damaged tissues has generated immense excitement. The field is now undergoing a more realistic and critical period of investigation, and cell fusion has been shown after specific types of damage in liver and muscle, both organs with a high number of multinucleate cells. The field is still an extremely exciting one, and many questions remain to be answered before Stem Cell Therapy for tissue repair can or should be used clinically. During our first period of funding, we developed the in vivo models necessary to now answer specific questions regarding tissue repair by stem cells. Our proposal will focus on using these models to address the three major questions in the field;1. What cell type can best mediate repair in liver and muscle? 2. Is repair occurring through fusion, differentiation, or secretion of angiogenic factors by infused stem cells to initiate revascularization? 3. How are stem cells recruited to sites of injury and can we enhance this to induce more robust repair? HGF is a chemoattractant and viability factor elevated in injured liver, cardiac, and skeletal muscle, in addition to other organs. It is activated by cleavage from the inactive form at sites of hypoxic damage. HGF affects the motility and maintains the viability of myoblasts, hepatic oval cells, and hematopoietic stem cells. We hypothesize that HGF, activated in response to tissue injury and hypoxia, acts in a gradient-specific manner to recruit primitive, c-met+ stem cells into the site of muscle or liver injury. We hypothesize that HGF will maintain the viability of the recruited stem cells, while they are directed by inductive, tissue-specific factors in the local microenvironment to differentiate, fuse, or initiate revascularization, to participate in tissue regeneration. To test this hypothesis, in Specific Aim 1, we will define the optimal human cell population to inject into immune deficient mice to induce the most robust recruitment to damaged liver vs. cardiac muscle. Purified HSC and MSC populations will be compared to the ALDH hi/lin- population, which contains both HSC and endothelial progenitors, and to HSC/MSC co- transplantation. Cell tracking to the injured tissue will be accomplished by GUSB expression from normal human cells in NOD/SCID/MPSVII (GUSB null) mice, coupled with immunohistochemistry and in situ hybridization. In Specific Aim 2, we will explore the role of hypoxia, HGF/c-met, and CXCR4/SDF-1 in recruitment of human progenitors to injured liver and muscle. In specific aim 3, we will use clonal marking at the single cell level to rigorously determine whether individual stem cells isolated from human cord blood and bone marrow can give rise to liver or muscle tissue, in addition to blood. Our Overall Goal is to optimize recruitment of defined human progenitor populations to damaged liver and muscle in novel immune deficient mouse models of tissue injury, to obtain more robust repair.